Infrared spectrometer

ABSTRACT

Method and apparatus for detecting, by absorption spectroscopy, an isotopic ratio of a sample, by passing first and second laser beams of different frequencies through the sample. Two IR absorption cells are used, a first containing a reference gas of known isotopic ratio and the second containing a sample of unknown isotopic ratio. An interlacer or reflective chopper may be used so that as the laser frequencies are scanned the absorption of the sample cell and the reference cell are detected alternately. This ensures that the apparatus is continuously calibrated and rejects the baseline noise when phase sensitive detection is used.

CROSS REFERENCE

This application is a Continuation-in-part of co-pending applicationSer. No. 11/606,084 filed on Nov. 30, 2006, the entire contents of whichare hereby incorporated by reference and for which priority is claimedunder 35 U.S.C. § 120; and this application claims priority, under 35U.S.C. § 120 and § 365(c), of PCT application PCT/GB2008/000539 filed on15 Feb. 2008, the entire contents of which are hereby incorporated byreference.

FIELD OF THE INVENTION

The present invention relates to a infrared, IR, spectrometer and inparticular to a tunable IR laser spectrometer.

The present invention also relates to a method of isotopic ratiodetermination and a method of selecting a pair of isotopic spectroscopiclines for a sample suitable for use in isotopic ratio determination.

BACKGROUND OF THE INVENTION

Stable isotopomer concentration and flux measurements have become avital underpinning technique in many areas of science. To date, massspectrometry (MS) has been the preferred technique to perform isotopeanalysis. For any application requiring real time, high frequency data,and/or field measurements, MS suffers from drawbacks: tedious sampletaking and preparation, difficulty of real time analysis. Furthermore,MS does not resolve small mass differences particularly well. The use ofMS is confined to a dedicated laboratory and field applications aretherefore difficult. There is a growing need to overcome these drawbacksand to seek alternative instrumentation for high precision (˜0.1‰)isotopic ratio determination both in the academic and industrialsectors. Applications encompass atmospheric studies, geology, ecology,medical research, planetary exploration, combustion science, fundamentalanalytical chemistry, and food industry quality check.

Therefore, there is requirement for a method and apparatus for providinghigh precision isotopomer concentration and ratio measurements withoutthe above drawbacks.

SUMMARY OF THE INVENTION

Generally, the present invention provides a method and apparatus fordetecting, by absorption spectroscopy, an isotopic ratio of a sample, bypassing first and second laser beams of different frequencies throughthe sample. In a particular example, this may be applied to thedetermination of ¹²CO₂:¹³CO₂ or similar.

Typically, two IR absorption cells are used, a first containing areference gas of known isotopic ratio and the second containing a sampleof unknown isotopic ratio.

Advantageously, an interlacer, switcher or reflective chopper may beused so that as the laser frequencies are scanned the absorption of thesample cell and the reference cell are detected alternately. Thisensures that the apparatus is continuously calibrated and rejects thebaseline noise when phase sensitive detection is used.

Preferably, the rate at which the sample cell and reference cell arealternatively detected is faster than several hundred Hz and even morepreferably, above 1 KHz. This improves the signal-to-noise of thesystem. The wavelength of the lasers may be scanned in synchronisationwith this alternate detection frequency. For instance, the full range(or a range of wavelengths between particular absorption bands of thesample and reference) of laser wavelengths may be scanned between eachdetection cycle. Phase sensitive detection may be used to maintain thissynchronisation.

According to a further aspect of the present invention there is provideda method of selecting a pair of isotopic spectroscopic lines for asample suitable for use in isotopic ratio determination comprising thestep of: (a) selecting a pair of isotopic spectroscopic lines of similarintensities such that a suitable pair of isotopic spectroscopic lines isselected.

Preferably, the method further comprises the step of: (b) comparing thethermal characteristics of said pair of isotopic spectroscopic lines andrejecting said pair if the thermal characteristics are significantlydifferent.

Advantageously, the thermal characteristics are determined to besignificantly different if the line intensity or wavelength variessignificantly with temperature.

Preferably, the isotopic spectroscopic lines are spaced far apart.

Preferably, the sample is selected from the group consisting of¹²CO₂/¹³Co₂, C¹⁶O₂/¹⁶O¹⁸, H₂ ¹⁶O/H₂ ¹⁸O, ¹²CH₄/¹³CH₄, ¹²CH₄/¹²CDH₃,¹⁴N₂O/¹⁴N¹⁵NO and H₂O/HDO

According to a further aspect of the present invention there is provideda method for detecting an isotopic ratio of a sample comprising thesteps of (a) passing a first laser beam and a second laser beam ofdifferent frequencies through a sample; and (b) detecting the opticalabsorption due to the sample such that a first absorption line and asecond absorption line are measured.

According to a further aspect of the present invention there is provideda method for detecting an isotopic ratio of a sample by measuring therelative intensities of at least one of the following pairs ofabsorption lines: 3601.4210 cm⁻¹/2294.4811 cm⁻¹, 3599.7027cm⁻¹/2295.8456 cm⁻¹, and 3597.9626 cm⁻¹/2297.1862 cm¹.

Further details of the present invention are described in theaccompanying claims.

BRIEF DESCRIPTION OF THE FIGURES

The present invention may be put into practice in a number of ways andan embodiment will now be described by way of example only and withreference to the accompanying drawings, in which:

FIG. 1 shows a schematic diagram of an IR laser spectrometer accordingto a first embodiment of the present invention, given by way of exampleonly;

FIG. 2 shows a timeline of the data acquired from the IR laserspectrometer of FIG. 1;

FIG. 3 shows a flow-chart of a method of collecting data from the IRlaser spectrometer of FIG. 1;

FIG. 4 shows an IR laser spectrometer according to a second embodimentof the present invention, given by way of example only;

FIG. 5 shows a set of graphs depicting pairs of suitable spectral linesto be investigated by the IR laser spectrometer of FIG. 1 or FIG. 4;

FIG. 6 a shows a graph illustrating conventional phase sensitivedetection in tunable spectroscopy using a chopper; and

FIG. 6 b shows a graph illustrating the signal obtained using the IRlaser spectrometer of FIG. 1.

DETAILED DESCRIPTION OF AN EMBODIMENT

The inventor of the present invention has described carbon isotopomerratio measurements in “Carbon isotopomers measurement using mid-IRtunable laser sources” Damien Weidmann, et al, Isotopes in Environmentaland Health Studies, Vol. 41, No. 4, December 2005, pp 293-302 (DOI:10.1080/10256010500384325), which is incorporated by reference in itsentirety herein.

Absorption spectroscopy techniques, using infrared laser sources, offeran improvement over MS. A useful spectral region for absorptionspectroscopy is known as the “fingerprint” region lying in the midinfrared range (2-25 μm), where most molecules exhibit intensevibrational transitions.

As an example, identification and measurement of gases released byvolcanoes provide information on magmatic sources and so aidinterpretation of volcanic behaviour and eruption forecasting. As wellas information on real-time trace gas concentrations in volcanicemissions, isotopic ratio measurements of species including carbon,hydrogen, nitrogen, oxygen and sulphur provide additional informationconcerning the gas sources and their geochemical history. Stable carbonisotopomers, especially ¹²CO₂ and ¹³CO₂, are of particular interest.Carbon dioxide is typically the most abundant gas released by volcanoesafter H₂O. ^(13/12)CO₂ monitoring can be used as a tracer todiscriminate the sources contributing to observed CO₂ emissions.

FIG. 1 shows a schematic diagram of an IR spectrometer 10 according afirst embodiment of the present invention.

Diode laser DL is a room temperature tunable antimonide laser operatingaround 3601 cm⁻¹. Quantum cascade laser QCL is a room temperaturetunable laser operating around 2294 cm⁻¹. Off-axis parabolic mirrorOAPM1 collimates the beam from the QCL and this collimated beam iscombined with the beam from DL using a dichroic mirror DM1.

The combined beam is then split into two paths by a beam splitter BSbefore each portion of the beam enters a dual channel absorption cellproviding the sample-laser interaction volume. Mirror M1 directs one ofthe split beams into channel 20, whilst the portion passing through thebeam splitter enters channel 30 directly. The beams may be split 50:50or by any other ratio. Channel 20 contains a reference sample of knowncomposition and channel 30 contains a sample under investigation. Forinstance channel 20 may contain a mixture of ¹²CO₂/¹³CO₂ of knownproportion and channel 30 may contain a mixture of ¹²CO₂/¹³CO₂ ofunknown proportion. The pressure, flow rate and temperature of thesample and reference may be controlled and monitored during absorptionmeasurements.

As two wavelengths probe each cell channel, the system performsmeasurements over four spectroscopic channels.

Detector 1 and detector 2 detect transmitted laser light originatingfrom the DL and QCL respectively. Detector 1 and detector 2 are eachrespectively sensitive to the wavelengths provided by the DL and QCLlasers. Dichroic mirror DM2 separates the combined beam ensuring thatdetector 1 receives light originating from the DL and detector 2receives light originating from the QCL. Off-axis parabolic mirrorsQAPM1 and QAPM2 focus the collimated beams onto detectors 1 and 2respectively.

At the output of the dual channel absorption cell a 10 KHz chopper 40with reflective blades plays the role of an interlacer. Mirror M2directs the output of channel 20 onto the chopper blades. With thechopper 40 presenting an opening 50 towards the output of the dualabsorption cell the output of channel 20 (reference) passes straightthrough and is not directed towards the two detectors. However, theoutput from channel 30 (sample) also passes straight through the chopperand is directed onto the two detectors.

With the chopper 40 presenting a reflective blade towards the output ofthe dual absorption cell the output of channel 30 is blocked by the backof the mirror and the output from channel 20 (reference) is directedonto the two detectors. In this way detectors 1 and 2 sequentiallyreceive IR radiation transmitted through each channel 20, 30 in turn. Anacquisition unit collects and stores data from each of the twodetectors. The acquisition unit may be in turn connected to a computersystem for data manipulation and storage. Alternately, the computersystem itself may act as the acquisition unit.

The DL and QCL lasers are each tunable across a range of wavelengths. DLcontrol controls the output wavelength of diode laser DL. QCL controlcontrols the output wavelength of the QCL laser. The wavelengths of eachlaser are scanned across a particular range in order to obtain anabsorption spectrum from the reference and sample with intensity beingmeasured by the two detectors. The tuning of each laser is synchronisedsuch that a step change intensity occurs after each cycle (open,reflective blade) of the chopper. Alternatively, instead of a stepchange, a smooth continuous wavelength change may be made.

The alternate sampling of the reference cell 20 and sample cell 30provides an effective onboard calibration.

FIG. 6 a shows a schematic illustration of conventional phase sensitivedetection in tunable spectroscopy using a chopper. The signal peak topeak amplitude is S₀ and only a small variation dS of this amplitudecontains useful information.

FIG. 6 b shows a schematic illustration of the signal obtained from thepresent invention using a reflective chopper. The signal peak to peakamplitude is dS. AC levels of amplification can be much higher as onlythe change is monitored. Other drifting parameters cancel out using sucha technique with phase sensitive detection.

FIG. 2 shows the timeline showing the synchronisation between the lasertuning for each laser (having wavelength λ₁ and λ₂, respectively) andthe chopper state, i.e. which of cell 20 (reference) or 30 (sample) isbeing measured by detectors 1 and 2. Note that each laser wavelengthchanges at the same time once a reference/sample cycle has beencompleted. It is not necessary that the wavelength of each laser changesat the same rate. Alternatively, a smooth variation of wavelength mayoccur throughout the cycle.

Isotopic ratio is usually expressed in terms of delta value defined byequation 1:

$\begin{matrix}{\delta = {\left( {\frac{R_{X}}{R_{S}} - 1} \right) \times 1000}} & {{Equation}\mspace{20mu} 1}\end{matrix}$

where R_(x) denotes the ratio of the heavier isotope to the lighter onein the sample to be measured, and R_(s) refers to the ratio of acalibrated reference sample. Units are “per mil” or ‰.

FIG. 3 shows a flow diagram of a method of obtaining a relative deltavalue, δ_(rel) of a CO₂ sample. In this case:

$\begin{matrix}{\delta_{rel} = {\frac{P_{13}}{P_{S\; 12}} - 1}} & {{Equation}\mspace{20mu} 2}\end{matrix}$

FIG. 4 shows a QCL-based two channel spectrometer 200. This two channelspectrometer allows isotopomer measures to be made for certain pairs ofspectroscopic lines but has limited functionality when compared withspectrometer 10 of FIG. 1. The identified target lines for this sensormay be located at 2311.105566 cm⁻¹ (¹²CO₂, ν3) and 2311.398738 cm⁻¹(¹³CO₂, ν3), respectively. With this selection, the δ¹³C measurementsare insensitive to gas temperature variations. However, the lineintensities differ by a factor of about 100, and therefore a dual pathlength absorption cell is required. The sensor has already beendescribed in “Development of a compact quantum cascade laserspectrometer for field measurements of CO₂ isotopes”, Appl. Phys. B, 80,pp 255-260 (2005), incorporated by reference herein. The spectroscopicsource is a pulsed, thermoelectrically cooled, DFB QC laser excited by25 ns current pulses. A dual path length absorption cell 110 is anastigmatic Herriott cell modified to incorporate an additional shortpath. After exiting the cells, the two beams are directed to twothermoelectrically cooled HgCdZnTe detectors. The complete opticalsubsystem is housed inside a sealed Delrin case and purged with drynitrogen to avoid contamination from atmospheric CO₂ or corrosive gases.The associated sensor electronics are located below the opticalplatform. A gas control subsystem alternating the flow of sample andcalibration gases into the absorption cell is connected to theinstrument. The operating wavelength of the QCL obtained for this systemwas found to be ˜8 cm⁻¹ distant from the selected frequencies. Also, anunusually large threshold current (9 A) prevented the use of a fast(˜400 Hz) subthreshold current ramp to rapidly tune the laserwavelength. A ¹²CO₂ line at 2320.7501 cm⁻¹ and a ¹⁶O¹²C¹⁸O line at2320.4599 cm⁻¹ may be targeted with this device. The intensity ratio ofthese lines is ˜20 The extrapolated precision of δ¹³C is ˜1‰. Because asingle laser source is used in spectrometer 200 only close absorptionlines may be investigated. Furthermore, the use of a single cellrequires separate reference calibration once a sample has been purged.

Before spectroscopic measurements may be taken a suitable pair ofspectroscopic absorption lines must be identified. In order to avoid theneed for different path lengths within the absorption cells (such asused in spectrometer 200) lines of similar intensity may be chosen. Asthis technique relies on measuring absolute absorption line intensitiesthe overriding requirement is to avoid lines which behave differently totemperature changes. Ideally, the relative change in intensity for eachline in a pair when the temperature varies will be the same. With singlelaser spectrometers the choice of lines is limited to lines very closetogether. This leads to difficulty in resolving the lines and usuallyresults in large differences in absolute line intensities. However, withthe availability of the dual lasers system of the present inventionlines very far apart in wavelength terms may be used as a pair.

In a further aspect of the present invention there is provided a methodof choosing isotopic spectroscopic lines. This line selection method,which consists of choosing optimum absorption lines for the twoisotopomers under study, involves the consideration of several issues.

The line intensities have to be considered. Ideally, similar intensitieswould be sought so that the same cell can be used (i.e. same pathlength), and to limit potential non-linearities in the detection system.The line intensities should also be high enough to ensure the bestsensor sensitivity and to allow the use of a compact absorption cell.The temperature stability requirement depends on the Boltzmanndistribution of the two respective isotopic transitions. A similarevolution of the two line intensities with temperature is important tolimit sensitivity to temperature fluctuations. This condition isexpressed equation 3:

ΔT·ΔE=Δδ·k·T ²  Equation 3

where Δδ is the target accuracy on δ, k the Boltzmann constant, T thetemperature, ΔE the energy difference between the lower levels of thetwo transitions and ΔT the temperature variation. Hence, in terms ofspectroscopic parameters to obtain temperature insensitivity, the lowerenergy levels must be identical.

The potential spectral and collisional interferences by other specieshave to be considered. In particular, it is important to avoidinterference due to water vapour. Well isolated lines are preferred,making concentration retrievals easier. Other parameters may be dictatedby the spectroscopic source to be used. For example, ensuring that thetwo lines are within the available tuning range and that the sourcelinewidth matches the absorption widths of the selected isotopic linepair.

To implement line selection for any pair of isotopomers of any molecule,an algorithm based on the aforementioned method may be used. First,depending on the experimental conditions, the minimum line intensitiesto be considered may be defined (cutoff intensity, S_(min)). Then,precision parameters may be entered into the algorithm: the minimumacceptable temperature stability ΔT, the relative difference allowedbetween the two line intensities ΔS and the minimum frequency separationrequired between the two lines Δν. All the possible line pairs of thetwo selected isotopomers within the available database are thenprocessed. The identified line pairs then have to be criticallyexamined, taking into account the other aforementioned selectioncriteria.

The algorithm may be applied to potential ¹²CO₂, ¹³CO₂ line pairs or anyother suitable molecule. The minimum intensity may be set to 10-21cm⁻¹/molec cm⁻². This intensity criterion is low for atmosphericstudies, but sufficient for volcanic environments where CO²concentrations can exceed several percent.

The parameters may be chosen as follows: a target delta value accuracyof 0.1‰, a temperature stability of better than 0.5 K, a relativedifference in intensities of <10% and line separation of at least 0.05cm⁻¹. A stringent condition on line intensities is required to achievethe expected accuracy on the delta value.

FIG. 5 shows the results of applying the above algorithm to thespectroscopic lines of ¹²CO₂ and ¹³CO₂. The lower graph provides detailsof the three ro-vibrational band intersections appearing in the maingraph: (ν3, ν3), (ν1, ν3), (2ν1+ν3, ν2), from left to right. One hundredand eighty-eight pairs of transitions satisfy these criteria. The pointsare located in three regions, detailed in the lower part of FIG. 5,corresponding to the intersection of the ¹²CO₂ and ¹³CO₂ bands indicatedon the upper graph.

FIG. 5 indicates that the optimum selection involves two lines from twodifferent bands, thus requiring the two laser sources of the presentinvention. With the parameters used to establish this selection, onlytwo pairs belong to the same band (ν3) and the smallest frequencyseparation is found to be ˜25 cm⁻¹ for 2336.5590 cm⁻¹ (¹³CO₂) and2311.3990 cm⁻¹ (¹²CO₂). Such a tuning range is not accessible from asingle semiconductor mid-IR sources.

By choosing more stringent parameters such as S_(min)=10-20 cm⁻¹/moleccm⁻², ΔT=10K, ΔS=1%, only three line pairs are found. They are listed intable 1.

TABLE 1 Optimum line selection for ^(13/12)CO₂ ratio measurements. ¹²CO₂¹³CO₂ Frequency Strength (cm⁻¹/molec Frequency Strength (cm⁻¹/molec(cm⁻¹) cm⁻²) Band (cm⁻¹) cm⁻²) Band 3601.4210 3.513e−020 ν1 + ν32294.4811 3.547e−020 ν3 3599.7027 3.573e−020 ν1 + ν3 2295.84563.574e−020 ν3 3597.9626 3.525e−020 ν1 + ν3 2297.1862 3.499e−020 ν3

These three line pairs offer the best strategy to achieve a highaccuracy for δ¹³C measurements.

The line pair selection may be made so that both the two intensities andthe two absorption frequencies are close. Inherently, the lower stateenergies of the selected line pair are different, ΔE ˜1160 cm⁻¹. Theexpected required temperature stability, calculated with equation 3, is6 mK. As a consequence, pressure change has a strong effect on the peakamplitude and overlapping wing contributions from one line to the other.

The present invention may be used in:

Geophysical studies: Real time continuous monitoring of isotopomersreleased by volcanic fumes. The knowledge of isotope concentrationsallows the deduction of information about the different sources feedinga volcanic system. The present invention strengthens the reliability ofvolcanic eruption forecasting. Also oil prospecting can be facilitatedby identifying carbon sources using this method.Atmospheric & environmental sensing: The present invention allows theprovision of high temporal and high spatial resolution ¹³C/¹²C data,thus providing further constraint to carbon budget models. This allows afuller understanding and account for carbon sources and sinks.Planetary exploration: The present invention may be used in planetaryexploration, especially Mars exploration.Medical diagnostics: Real time, continuous monitoring of trace moleculeenables non-invasive breath diagnostics. For instance, the activity ofPylobacter Pylori (bacteria responsible of stomach ulcer) may beidentified by the measurement of ¹²CO₂/¹³CO₂ ratio in breath. ¹²C/¹³Cratio may also discriminate between a catabolic and anabolic state ofliving cells.Agronomy and food industry: Isotopomers concentration contained in agiven product may indicate the product origin. Real time, continuousmonitoring of isotopic ratio by the present invention may provideon-line implementation, and detection of artificial substitutes.

As will be appreciated by the skilled person, details of the aboveembodiments may be varied without departing from the scope of thepresent invention.

For example, the chopper may be placed before the absorption cells 20,30 to alternately block or switch the incident IR beams rather thanblocking the transmitted beams. In this alternative embodiment the beamsplitter may not be required resulting in all of the available laserintensity being incident on one cell at a time. Obviously, the detectorarrangement will require adjustment accordingly.

A single detector may be used provided that it can detect bothwavelengths.

Although a 10 kHz chopper frequency has been described, any suitablefrequency may be used.

Preferred embodiments and methods of the present invention discussed inthe foregoing are to be understood as descriptions for illustrativepurposes only, and it will be appreciated that numerous changes,substitutions, omissions, and updates thereof are possible withoutdeparting from the spirit and scope of the claims.

1. An infrared, IR, spectrometer comprising: a first tunable IR laserfor providing a first beam at a first wavelength; a second tunable IRlaser for providing a second beam at a second wavelength; a firstabsorption cell arranged to receive both first and second beams; asecond absorption cell arranged to receive both first and second beams;a first optical absorption detector arranged to detect said firstwavelength; and a second optical absorption detector arranged to detectsaid second wavelength.
 2. The IR spectrometer of claim 1 furthercomprising: an interlacer arranged to alternately block said first andsaid second absorption cells.
 3. The IR spectrometer of claim 2, whereinsaid interlacer is an optical chopper comprising at least one reflectiveblade angled to direct transmitted IR radiation from one of said opticalabsorption cells on to both of said optical absorption detectors.
 4. TheIR spectrometer according to claim 2, wherein the wavelengths of saidfirst and second tunable IR lasers are changeable after said first andsaid second optical absorption detectors have been blocked.
 5. The IRspectrometer according to claim 3, wherein the first and second IRtunable lasers are arranged to vary the first and second wavelengths insynchronisation with the optical chopper.
 6. The IR spectrometer ofclaim 5, wherein the first and second wavelengths are arranged to varystepwise.
 7. The IR spectrometer of claim 5, wherein the first andsecond wavelengths are arranged to vary smoothly.
 8. The IR spectrometerof claim 6, wherein the first and second wavelengths are arranged to bevaried across a selection of absorption lines of a sample beingmeasured.
 9. The IR spectrometer of claim 5, wherein the optical chopperis arranged to direct transmitted IR radiation from one of said opticalabsorption cells on to both of said optical absorption detectors at afrequency.
 10. The IR spectrometer of claim 9, wherein the first andsecond optical absorption detectors are arranged to detect at thefrequency.
 11. The IR Spectrometer of claim 10, wherein the detection ismaintained at the frequency by phase detection of the chopper frequency.12. The IR Spectrometer according to any of claims 2, wherein theinterlacer alternatively blocks said first and said second absorptioncells at or above 1 KHz.
 13. The IR spectrometer of claim 1 furthercomprising a beam combiner arranged to combine said first beam with saidsecond beam to form a combined beam such that said first absorption celland said second absorption cell each receive a portion of said combinedbeam.
 14. The IR spectrometer of claim 1, wherein said beam combinercomprises a dichroic mirror.
 15. The IR spectrometer of claim 1, furthercomprising a beam splitter arranged to direct a portion of the combinedbeam into each of said first and second absorption cells.
 16. The IRspectrometer of claim 1 further comprising a beam separator arranged toseparate said first wavelength from said second wavelength such thatsaid first wavelength is detected by said first detector and said secondwavelength is detected by said second detector.
 17. The IR spectrometerof claim 1, wherein said first tunable IR laser is a diode laser. 18.The IR spectrometer of claim 17, wherein said diode laser is tunablebetween about 2 μm and about 12 μm.
 19. The IR spectrometer of claim 1,wherein said second tunable IR laser is a quantum cascade laser.
 20. TheIR spectrometer of claim 19, wherein said quantum cascade laser istunable between about 4 μm and about 24 μm.
 21. The IR spectrometer ofclaim 1 further comprising a data logger for recording data generated bysaid first and second optical absorption detectors.
 22. The IRspectrometer of claim 1, wherein said first absorption cell contains areference sample and said second absorption cell contains a sample. 23.The IR spectrometer of claim 1, wherein the first wavelength isdifferent to the second wavelength.
 24. The IR spectrometer of claim 1,wherein said first and second absorption cells contain a sampleconsisting of one of ¹²CO₂/¹³CO₂, C¹⁶O₂/¹⁶OC¹⁸O, H₂ ¹⁶O/H₂ ¹⁸O,¹²CH₄/¹³CH₄, ¹²CH₄/¹²CDH₃, ¹⁴N₂O/¹⁴N¹⁵NO and H₂O/HDO


25. An infrared, IR, spectrometer comprising: a first tunable IR laserfor providing a first beam at a first wavelength; a second tunable IRlaser for providing a second beam at a second wavelength; a firstabsorption cell arranged to receive both first and second beams; asecond absorption cell arranged to receive both first and second beams;a first optical absorption detector arranged to detect said firstwavelength; a second optical absorption detector arranged to detect saidsecond wavelength; and an interlacer arranged such that said first andsaid second absorption cells are alternately switched from receivingsaid first and said second beams.
 26. A method of isotopic ratiodetermination comprising the steps of: (a) passing a first tunable IRlaser beam through a first absorption cell; (b) passing a second tunableIR laser beam through a second absorption cell; (c) detecting theabsorption due to a first sample contained within said first cell; and(d) detecting the absorption due to a second sample contained withinsaid second cell such that the isotopic ratio of said second sample isdetermined.
 27. The method of claim 26 further comprising the step of:(ii) alternately switching said first and second IR laser beams suchthat the detecting steps (c) and (d) occur in turn.
 28. The method ofclaim 26 further comprising the step of: (i) combining said firsttunable IR laser beam with said second tunable IR laser beam beforepassing both laser beams through both cells.
 29. The method of claim 26further comprising the step of varying the wavelength of each tunable IRlaser beam following each sequence of step (c) and (d).
 30. The methodof claim 29, wherein the wavelength of each tunable IR laser is variedstepwise.
 31. The method of claim 29, wherein the wavelength of eachtunable IR laser is varied smoothly.
 32. The method of claim 29, whereinthe wavelength is varied across a selection of absorption lines of asample being measured.
 33. The method of claim 29, wherein the first andsecond IR laser beams are alternatively switched at a frequency anddetecting the absorption due to the first and second samples occurs atthe frequency.
 34. The method of claim 33, wherein the detection ismaintained at the frequency by phase detection.
 35. The method of claim33, wherein the frequency is at or above 1 KHz.
 36. A method ofselecting a pair of isotopic spectroscopic lines for a sample suitablefor use in isotopic ratio determination comprising the step of: (a)selecting a pair of isotopic spectroscopic lines of similar intensitiessuch that a suitable pair of isotopic spectroscopic lines is selected.37. The method of claim 36 further comprising the step of: (b) comparingthe thermal characteristics of said pair of isotopic spectroscopic linesand rejecting said pair if the thermal characteristics are significantlydifferent.
 38. The method of claim 37, wherein the thermalcharacteristics are determined to be significantly different if the lineintensity or wavelength varies significantly with temperature.
 39. Themethod of claim 37, wherein the isotopic spectroscopic lines are spacedfar apart.
 40. The method of claim 36, wherein the sample is selectedfrom the group consisting of ¹²CO₂/¹³CO₂, C¹⁶O₂/¹⁶OC¹⁸O, H₂ ¹⁶/H₂ ¹⁸O,¹²CH₄/¹³CH₄, ¹²CH₄/¹²CDH₃, ¹⁴N₂O/¹⁴N¹⁵NO and H₂O/HDO


41. A method for detecting an isotopic ratio of a sample comprising thesteps of (a) passing a first laser beam and a second laser beam ofdifferent frequencies through a sample; and (b) detecting the opticalabsorption due to the sample such that a first absorption line and asecond absorption line are measured.
 42. A method for detecting anisotopic ratio of a sample by measuring the relative intensities of atleast one of the following pairs of absorption lines: 3601.4210cm⁻¹/2294.4811 cm⁻¹, 3599.7027 cm⁻¹/2295.8456 cm⁻¹, and 3597.9626cm⁻¹/2297.1862 cm⁻¹.